The present invention relates to a device for optically measuring the position of an object and, more particularly, to a device suitable for measuring the height of an object.
In an LSI manufacturing device such as an electron beam exposure device, the surface level of the object may vary with respect to the optical system due to warping of the sample or for other reasons when a pattern is drawn on a semiconductor wafer or a mask substrate. An error in the drawn pattern thus occurs. A conventional technique is employed to measure the variation along a direction perpendicular to the surface of the sample to correct the variation.
A technique using a device for measuring the surface level of an object is disclosed in Japanese Patent Laid-Open Publication (Kokai) No. 56-2632. With this device, light emitted from a laser source is focused to form a beam spot on the sample surface. Light reflected from the sample surface is focused by another lens on a semiconductor position sensor which uses a lateral photo effect. The detection output from the sensor is processed to measure the surface level of the sample.
In a surface level measuring device of this type, however, there is the following problem. When the sample surface has a portion with high reflectance and a portion with low reflectance, the intensity distribution of the beam is unbalanced due to the difference in reflectance as shown in FIG. 1, resulting in a measurement error. For example, when a PSD (Semiconductor Position Detector) which has been recently used in a variety of applications to measure the barycenter of the irradiation light intensity is used, the barycenter varies due to different beam intensities (FIG. 2A), as shown in FIG. 2B, thus resulting in a measuring error. On the other hand, when a vibration slit technique for a photoelectric microscope is used, a measuring error occurs due to nonuniform distribution of the beam spot.